1. Field of the Invention
This invention relates generally to pixel driver circuits for active-matrix organic light-emitting diode (AMOLED) displays and microdisplays, and more particularly, to such a circuit that includes active temperature control through the use of an on-chip heater to reduce the level of the voltage bias necessary to operate the display over a wide temperature range, especially below 0° C., and at high luminance levels.
2. Description of the Related Art
Organic light emitting diode (OLED) devices are known to shift their current-voltage characteristics dramatically over temperature, with a large increase occurring at temperatures below 0° C. As a result, the silicon backplane must handle drive voltages that exceed the capability of typical CMOS technology. This is currently accomplished by applying a portion of the required bias voltage, namely the negative cathode supply, to all the pixels in common and tied to the silicon substrate ground potential. This approach provides the ability to operate AMOLED microdisplays over a wide temperature range (−50 to 70° C.) but at the expense of design complexity, additional external components, and limitations to the pixel size reduction.
The use of an on-chip heater to raise the OLED temperature rapidly above 0° C. at very low ambient temperatures will eliminate the need for a negative cathode supply. The heating can be supplied by applying power to a resistive thin-film located above the OLED layer, such as an indium-tin-oxide (ITO) layer, and regulating the power according to the ambient temperature.
While these units may be suitable for the particular purpose employed, or for general use, they would not be as suitable for the purposes of the present invention as disclosed hereafter. Specifically, the present invention raises the OLED temperature by dissipating power in actively controlled devices found in the silicon drive circuitry located below the OLED layer, such as the BIAS transistor used in the voltage pixel driver of the preferred embodiment.
It is, therefore, a primary object of the present invention to operate an AMOLED microdisplay over a wide temperature range (−50 to 70° C.) using the present and advantageous circuit design architecture without compromising design complexity, adding external components or limiting the pixel size reduction.
It is, therefore, a primary object of the present invention to provide a pixel circuit that reduces the voltage bias over a wide temperature range, especially below 0° C., and at high luminance levels.
It is, therefore, another object of the present invention to reduce the complexity and cost of the silicon backplane and external electronics used for the AMOLED microdisplay.
It is, therefore, another object of the present invention to provide a path to further miniaturization of the pixel drive circuit.
It is, therefore, another object of the present invention to operate the circuit over a more limited temperature range for the OLED, such that the optical performance of the AMOLED microdisplay is significantly improved.